It is still unfortunate that the external quantum efficiency of ultraviolet (UV) light-emitting diodes (LEDs) is still much lower in comparison to that of blue and green light-emitting diodes. Regardless, the former have been widely applied in water purification systems, optical communications and as chemical sensors for the detection of biohazardous and heavy metals. This unwanted shortcoming is due to the presence of AlGaN layers in the UV LEDs active layers which strongly influences the non-radiative recombination process via the high density of threading dislocations and polarization fields related to the quantum-confined stark effect. The consequence of the latter is the decreased internal quantum efficiency which translates to low external quantum efficiency. To resolve this drawback, it is important to enhance the internal quantum efficiency in addition to the light extraction efficiency of UV LEDs.
A team of researchers led by Professor Tae Geun Kim from the School of Electrical Engineering at Korea University proposed a study whose main objective was to cross-examine the effect of an aluminum nanoring structure on the light extraction efficiency from AlGaN/GaN multiple quantum wells of 365nm LEDs through the localized surface plasmons coupling effect. They hoped that their technique would result in simultaneous enhancement of both the internal quantum efficiency and external quantum efficiency. Their work is published in the research journal, Photonics Research.
The experiments employed commenced by investigating the localized surface plasmons resonance absorption spectra for different size of aluminum nanorings. Next, the researchers selected an appropriate nanoring size that could interact with the light emitted from the 365 nm LEDs so as to confirm that the photoluminescence intensity was increased. Eventually, the team conducted time-resolved photoluminescence measurements and temperature dependent photoluminescence measurements so as to examine the carrier dynamics of the recombination mechanism.
The researchers observed that from the temperature dependent photoluminescence and time-resolved photoluminescence measurements, the calculated photoluminescence efficiencies at room temperature for the structure with and without aluminum nanorings were 33% and 19%, respectively. Additionally, they also noted that the computed radiative lifetimes were 0.63 and 1.24 ns for the structure with and without aluminum nanorings, respectively, at room temperature.
Tae Geun Kim and his colleagues presented significant enhancement of internal quantum efficiency of LEDs that have a wavelength of 365 nm using localized surface plasmons – quantum wells. This has been achieved due to the fabrication of aluminum nanoring arrays on top of an AlGaN/GaN multiple quantum wells structure. The improvement recorded here can mainly be attributed to the enhanced radiative recombination rate in the multiple quantum wells through the energy-matched localized surface plasmons by the temperature-dependent photoluminescence and time-resolved photoluminescence analyses. Altogether, both localized surface plasmons – quantum wells coupling and the nanoring array pattern play vital roles in the enhancement.
Kyung Rock Son, Byeong Ryong Lee, Min Ho Jang, Hyun Chul Park, Yong Hoon Cho, Tae Geun Kim. Enhanced light emission from AlGaN/GaN multiple quantum wells using the localized surface plasmon effect by aluminum nanoring patterns. Volume 6, No. 1 / 2018 / Photonics Research
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